Ceramic block filters offer several advantages over lumped component filters because they are relatively easy to manufacture, rugged, and relatively compact. In the basic ceramic block filter design as shown in, for example, U.S. Pat. No. 4,431,977 to Sokola et al. and U.S. Pat. No. 6,559,735 to Hoang and Vangala, resonators are formed by cylindrical passages, called through-holes, which extend through the block. The block is substantially plated with a conductive material (i.e., metallized) on all but one of its six (outer) sides and on the inside walls formed by the resonator holes.
One of the two opposing sides containing holes is not fully metallized, but instead bears a metallization pattern designed to couple input and output signals through the series of resonators. The reactive coupling between adjacent resonators is dictated, at least to some extent, by the physical dimensions of each resonator, by the orientation of each resonator with respect to the other resonators, and by aspects of the top surface metallization pattern.
Although these types of RF filters have received widespread commercial acceptance for RF frequencies of 5.8 GHz or lower, efforts at improving this basic design for RF frequencies above that level has continued. Specifically, and in the interest of allowing wireless communication providers to provide additional service, governments worldwide have allocated new higher RF frequencies for commercial use in, for example, wireless base stations, WiMax base stations, WLAN, and UWB (ultra wideband) consumer and infrastructure products. To better exploit these newly allocated frequencies, standard-setting organizations have adopted bandwidth specifications with compressed transmit and receive bands as well as individual channels. These trends are pushing the limits of currently available filter technology to provide sufficient frequency selectivity and band isolation.
The need thus continues for improved ceramic RF filters which can offer selectivity and other performance improvements at 5.8 GHz and higher, all in a lower cost, smaller, higher performance, and lower height package than the ceramic filters available today. This invention provides such an improved ceramic filter.